A transformer is an electronic assembly, which is frequently used in various electronic apparatuses for modulating different voltages in the range applicable to the power-receiving apparatuses. Referring to FIG. 1, a conventional transformer 1 includes a bobbin 11, a coil set 12, a pin seat 14 and a core 15. The coil set 12 includes a primary coil and a secondary coil each formed by winding a conductive wire around a winding region 111 of the bobbin 11. The primary coil is electromagnetically coupled with the secondary coil. The pin seat 14 is connected with the bobbin 11 and includes a plurality of pins 141, which is for electrically connecting with a circuit board 17. The core 15 is composed of a first core 151 and a second core 152, which are respectively fit with two ends of the bobbin 11 and are connected together to form an electromagnetic loop.
In general, the coil set is made in the form of concentric or stacked winding. The concentric coil set 12 is directly wound around the bobbin 11, while the stacked winding structure has a primary coil and a secondary coil each wound to form a wire cake 20, as shown in FIG. 2, and then the wire cakes 20 are alternately arranged along an axial direction of the bobbin 11 and then fit with the bobbin 11. The conventional wire cake 20 is a single-layer spiral wire cake formed by spirally winding a conductive wire outward on a plane, and both ends (i.e., a beginning end 20a and a terminating end 20b) of the conductive wire are suspended outside the structure of the wire cake 20 for being electrically connected with a pin or connected to a power source. However, under large capacity and high current requirements for the transformer 1, the number of the coils is typically increased, but it takes time and cost to connect each wire cake 20.
Besides, the electronic device trends to be small and thin nowadays, the volume of the transformer has to be reduced, and the structure of the transformer needs to be simplified to facilitate assembling, application and heat-dissipation. Therefore, it is an important object to provide a transformer to achieve the foresaid objects.